Nonaqueous electrolyte batteries such as lithium secondary batteries have been putting into practical use for various purposes, for example, from so-called household power sources for mobile phones, notebook computers, and so on to driving batteries equipped on vehicles such as automobiles.
However, recent requirements for high-performance nonaqueous electrolyte batteries have become higher and higher, and battery characteristics are desired to be improved.
In general, the electrolytic solution used in a nonaqueous electrolyte battery is mainly composed of an electrolyte and a nonaqueous solvent. As the main component of the nonaqueous solvent, for example, a cyclic carbonate such as ethylene carbonate or propylene carbonate, a chain carbonate such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate, or a cyclic carboxylate such as γ-butyrolactone or γ-valerolactone is used.
In addition, in order to improve the battery characteristics, such as load characteristics, cycle characteristics, and storage characteristics, of these nonaqueous electrolyte batteries, various investigations of nonaqueous solvents and electrolytes have been performed.
For example, Patent Document 1 proposes an electrolytic solution containing a phosphinic ester for producing a battery in which degradation in battery performance during high-temperature storage is suppressed.
Patent Document 2 proposes an electrolytic solution containing a phosphonoacetate for producing a nonaqueous electrolyte secondary battery that is safe because of its high flame retardancy and can generate a high voltage and also is excellent in charge and discharge.
Patent Document 3 proposes an electrolytic solution containing a phosphonoacetate for producing a battery excellent in flame retardancy.
Patent Document 4 proposes the use of a mixture of an asymmetric chain carbonate and a cyclic carbonate having a double bond as a nonaqueous solvent. By using this mixture, the cyclic carbonate having a double bond preferentially reacts with a negative electrode to form a high-quality film on the surface of the negative electrode, and thereby the asymmetric chain carbonate is prevented from forming a nonconductive film on the surface of the negative electrode, resulting in enhancements in storage characteristics and cycle characteristics.
However, recent requirements for high-performance batteries have become higher and higher, and it is required to achieve high capacity, high-temperature storage characteristics, and cycle characteristics at high levels.
As a method of increasing the capacity, it has been investigated to fill the restricted battery content with an active material in an amount as large as possible. In general, the active material layer of an electrode is pressurized to increase the density, or a battery is designed such that the volume of materials other than the active material in the battery is reduced as much as possible. However, when the active material layer of an electrode is pressurized to increase the density or the amount of an electrolytic solution is decreased, the active material cannot be uniformly used and degradation of the active material is accelerated by uneven reaction. This tends to cause a problem in which sufficient characteristics cannot be obtained. Furthermore, an increase in the capacity of a battery causes a decrease in the space inside the battery. This also causes a problem in which the internal pressure of the battery is significantly increased even if the amount of gas generated by decomposition of the electrolytic solution is small.
In particular, in most cases using a nonaqueous electrolyte secondary battery as a power supply in case of power outage or as a power supply of portable equipment, the battery always supplies an extremely weak current for compensating the self-discharge of the battery and is always in a state of charging. In such a continuous charging state, the electrode active material is always in a high activity state, and, simultaneously, heat generated by the equipment accelerates a decrease in the capacity of the battery or decomposition of the electrolytic solution to tend to generate gas. In a battery that opens a relief valve when an abnormal increase in the internal pressure, due to abnormality such as overcharge, is detected, the relief valve may be falsely opened if a large amount of gas is generated. In a battery not having a relief valve, the battery expands by the pressure of the generated gas, and the battery itself may become disabled.
The nonaqueous electrolyte secondary battery including the electrolytic solution described in Patent Document 1 is still unsatisfactory in suppression of gas generation, high-temperature storage characteristics, and cycle characteristics, as described above.
In the nonaqueous electrolyte secondary battery including the electrolytic solution described in Patent Document 2 or 3, since the phosphonoacetate contained in the electrolytic solution has a hydrogen atom on the α-position of the carbonyl group, it is suggested that elimination reaction of hydrogen tends to occur in reductive decomposition on the negative electrode side. Thus, battery characteristics are still unsatisfactory.
The nonaqueous electrolyte secondary battery including the electrolytic solution described in Patent Document 4 is also still unsatisfactory in suppression of the gas generation and high-temperature storage characteristics, as described above.
[Patent Document 1] Japanese Patent Publication 2004-363077A
[Patent Document 2] Japanese Patent Publication H10-189039A
[Patent Document 3] Japanese Patent Publication H11-233141A
[Patent Document 4] Japanese Patent Publication H11-185806A